We're using ParserOutput::setExtensionData('core:targetlang', ...) in core; this should be a new key in ParserOutput but we don't want to add lots of new top level properties! Instead we'd like to have a general key/value mechanism for core properties parallel to the setExtensionData mechanism used by extensions (T296345: Add ParserOutput::addOutputData() to unify various accumulators in ParserOutput).
This task is a placeholder to migrate the "unusual" use of ParserOutput::setExtensionData over to this mechanism once it exists.